There is the toleration of the philosopher to whom all religions are equally true; of the historian to whom all are equally false; and of the politician to whom all are equally useful. There is the toleration of the man who tolerates other modes of thought and behaviour because he has himself grown absolutely indifferent to all modes of thought and behaviour. There is the toleration of the weak man who, on account of sheer weakness, must pocket all kinds of insults heaped on things or persons that he holds dear.
Edward Gibbon is best known for his multi-volume book The History of the Decline and Fall of the Roman Empire (published from 1776 to 1788). I have not been able to check his writings for tracing this excerpt but it was quoted by Iqbal with attribution to Gibbon in 'Islam and Ahmedism', an article published in January 1936.